1. Field of the Invention
This invention relates to fire sensing systems and, more particularly, to such systems particularly designed to discriminate between stimuli from fire and non-fire sources.
2. Description of the Prior Art
Sensing the presence of a fire by means of photoelectric transducers is a relatively simple task. This becomes more difficult, however, when one must discriminate reliably between stimuli from a natural fire and other heat or light stimuli from a non-fire source. Radiation from the sun, ultraviolet lighting, welders, incandescent sources and the like often present particular problems with respect to false alarms generated in fire sensing systems.
It has been found that improved discrimination can be developed by limiting the spectral response of the photodetectors employed in the system. A plurality of signal channels having different spectral response bands have been employed in a number of prior art systems which utilize different approaches to solving the problem of developing suitably sensivity for fire sensing while reliably discriminating against non-fire stimuli.
The Cinzori U.S. Pat. No. 3,931,521 discloses a dual-channel fire and explosion detection system which uses a long wavelength radiant energy responsive detection channel and a short wavelength radiant energy responsive channel and imposes a condition of coincident signal detection in order to eliminate the possibility of false triggering. Cinzori et al U.S. Pat. No. 3,825,754 adds to the aforementioned patent disclosure the feature of discriminating between large explosive fires on the one hand and high energy flashes/explosions which cause no fire on the other. U.S. Pat. No. 4,296,324 of Kern and Cinzori discloses a dual spectrum infrared fire sensing system in which a long wavelength channel is responsive to radiant energy in a spectral band greater than about 4 microns of electromagnetic radiation and a short wavelength channel which is responsive to radiant energy in a spectral band less than about 3.5 microns, with at least one of the channels responsive to an atmospheric absorption wavelength which is associated with at least one combustion product of the fire or explosion to be detected. McMenamin, in U.S. Pat. No. 3,665,440, discloses a fire detector utilizing ultraviolet and infrared detectors and a logic system whereby an ultraviolet detection signal is used to suppress the output signal from the infrared detector. Additionally, filters are provided in series with both detectors to respond to fire flicker frequencies of approximately 10 Hz. As a result, an alarm signal is developed only if flickering infrared radiation is present. A threshold circuit is also included to block out low level infrared signals, as from a match or cigarette lighter, and a delay circuit is incorporated to prevent spurious signals of short duration from setting off the alarm.
Muller, in U.S. Pat. Nos. 3,739,365 and 3,940,753, discloses dual channel detection systems utilizing photoelectric sensors respectively responsive to different spectral ranges of incident radiation, the signals from which are filtered for detection of flicker within a frequency range of approximate 5 to 25 Hz. A difference amplifier generates an alarm signal in one of these systems when the signals in the respective channels differ by more than a predetermined amount from a selected value or range of value. In the other system, the output signals from the difference amplifier are applied to a phase comparator with threshold circuitry and delay. An alarm signal is provided only if the input signals are in phase, of amplitude in excess of the threshold level, and of sufficient duration to exceed the preset delay.
The Paine U.S. Pat. No. 3,609,364 utilizes multiple channels specifically for detecting hydrogen fires on board a high altitude rocket with particular attention directed to discriminating against solar radiation and rocket engine plume radiation.
The Muggli U.S. Pat. No. 4,249,168 utilizes dual channels respectively responsive to wavelengths in the range of 4.1 to 4.8 microns and 1.5 to 3 microns. Signals in both channels are subjected to a bandpass filter with a transmission range between 4 and 15 Hz for flame flicker frequency response. Both channels are connected to an AND gate so that coincidence of detection in both channels is required for a fire alarm signal to be developed. Other fire alarm or fire detection systems are disclosed in MacDonald U.S. Pat. No. 3,995,221, Schapira et al U.S. Pat. No. 4,206,454, McMenamin U.S. Pat. No. 3,665,440, Steele et al U.S. Pat. No. 3,122,638 and Krueger U.S. Pat. Nos. 2,722,677 and 2,762,033.
Despite the abundance of systems in the prior art for fire detection, the fact remains that no system has proved to be fully effective in discriminating against false alarms. In those systems where sensitivity is enhanced, there appears to be a concomitant degradation in other performance parameters, such as false alarm immunity. The present invention is directed to techniques for improving small fire detection sensitivity without sacrificing performance in other respects.